The going theory is that we’re fishing our way down the food chain across the globe as we overfish the larger, predator species (also known as high trophic-level) and move onto smaller stocks of baitfish. Goldstone points out in Cape Cod, a common example cited is how fishermen moved from on to dogfish and herring after cod stocks plummeted. On the West Coast, there has been an increase in sardine and whiting/hake fishing as salmon runs have declined.

But the question of how do we know if we are overfishing down the food chain is now the subject of debate, as Goldstone writes:

WHAT WE KNOW

If we were fishing our way down the marine food chain, catches of high-trophic-level fish should be dropping. Instead, the new study indicates that the average trophic levels of fish being caught declined in the 1970′s, but that catches of fish at all trophic levels have generally gone up since the mid-80s. Included are high-trophic predators such as bigeye tuna, skipjack tuna and blue whiting.

HOW WE KNOW

The current analysis is based on a comprehensive set of data – including worldwide catch data, stock assessments, scientific trawl surveys, small-scale fishery data and modeling results – compiled by the National Center for Ecological Analysis & Synthesis.

WHAT WE DON’T KNOW

This work brings into question the true status of many of the world’s fisheries – their health and sustainability (or lack thereof). Trevor Branch, a University of Washington assistant professor of aquatic and fishery sciences and the lead author of the study says the system currently in place is “like flipping a coin, half the time you get the right answer and half the time you get the wrong answer.” If fishermen targeted all levels of the food chain equally, entire ecosystems could be fished to collapse while the average trophic level of the catch remained steady – an indicator of fishery health under the current system. If fishermen targeted low-level shellfish before moving on to top predators, things could even look like their improving while they’re actually crashing. That’s exactly what has happened in the Gulf of Thailand, where fish at all levels have declined tenfold since the 1950s while the average trophic level indicator has been rising.”

To sum up the conundrum, Goldstone quotes Henry Gholz, program director for the National Science Foundation’s Division of Environmental Biology:

“Monitoring all the fish in the sea would be an enormous, and impossible, task. But this study makes clear that the most common indicator, average catch trophic level, is a woefully inadequate measure of the status of marine fisheries.”

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